1. Field of the Invention
The present invention relates to an optical disk drive apparatus usable for optimizing the focusing position of an optical pickup with respect to information recording surface of an optical disk medium.
2. Description of the Related Art
A conventional optical disk drive apparatus is, for example, disclosed in U.S. Pat. No. 5,090,003 (Feb. 18, 1992). An optical pickup includes: a laser diode; an objective lens; and a focus actuator, so that a concentrated laser beam is irradiated onto an optical disk. The optical pickup further includes a photo signal detector and a focus error detector. The photo signal detector reads information recorded on the optical disk via the objective lens, thereby outputting a read-out information signal. The focus error detector outputs a focus error signal having a level corresponding to the distance between a focusing position of the concentrated laser beam and the information recording surface of the optical disk. This focus error signal is fed back to the focus actuator through a focusing servo circuit, thereby maintaining the focus error constant. The position of the focus error detector is adjusted so that the focus error is "0", i.e., the focusing position is on the information recording surface, when the level of the focus error signal is "0".
However, the precision in locating the focus error detector is on the order of 10 .mu.m, so that the position of the detector sometimes deviates after a long-term use. In such a case, even if the level of the focus error signal is "0", "an error of the focusing error" may be generated (here, such an error is called a "focus offset"). As a result, the concentrated laser beam on the information surface of the optical disk becomes broad, and therefore the recorded information can not be identified so precisely any longer. Consequently, an error rate in the read-out information signal becomes disadvantageously higher.
In order to eliminate such an error, according to a conventional technique, the following focusing control apparatus is proposed. An offset compensating signal is supplied to the focusing control system. By varying the level of the offset compensating signal, the variation in the amplitude of the information read-out signal is detected, thereby searching for a point where the amplitude reaches the maximum level. When the point at the maximum level is found, the amount for compensating the offset at the point is fixed, and thereafter the fixed amount is applied constantly to the focusing servo circuit. As a result, the focal point of the concentrated laser beam may be substantially fixed on the recording medium surface of the optical disk.
According to the conventional technique, however, it is difficult to detect a focusing position with a high precision because of the following problems. Specifically, the amplitude of the information read-out signal does not vary so sharply in accordance with the variation of the focus error, and in addition, the amplitude of the information read-out signal is disturbed by the uneven reflective index over the recording surface of an optical disk or fluctuation of a light emitting power of a laser diode.